This invention relates to a recording apparatus for recording a grayscale image on a recording medium based upon input image data, as well as to a method of controlling this apparatus.
The growing popularity of copiers, information processors such as word processors and computers and communications equipment has given rise to the rapid spread of image forming (recording) devices for these apparatus, one example of such a device being a printer which performs digital image recording using an ink-jet recording head. In addition, the higher quality and colorization of visual information in the aforesaid information processors and communications equipment has been accompanied by increasing demand for higher image quality and colorization in recording apparatus as well.
In such a recording apparatus, use is made of a recording head (referred to as a "multihead" below) comprising a plurality of recording elements in an integrated array for the purpose of producing finer pixels, one example being an arrangement in which a plurality of ink discharge ports and ink passageways are integrated at high density. In order to produce colors, the general practice is to use an apparatus having a plurality of the aforesaid multiheads corresponding to the inks used, namely inks of the colors cyan, magenta, yellow and black.
There is a fixed limit, however, in the degree to which ink discharge ports and ink passageways can be densely integrated, and hence there is a fixed limit in the degree to which finer pixels can be produced. As a consequence, the dots which form the pixels are comparatively large and result in a grainy appearance at locations where there are image highlights having low density. This gives rise to problems in terms of improving image quality.
Instead of raising the density of integration of the ink discharge ports and ink passageways, i.e., instead of reducing the size of the individual pixels, it is known to employ a so-called multidrop technique through which the dots of the jetted ink are reduced so that each individual pixel will be formed by ink droplets the number of which conforms to the recording density. With the multidrop technique, the diameter of an ink dot recorded on recording paper can be made comparatively small, thus making it possible to diminish the graininess at low-density portions such as the highlight portions of an image. However, owing to a balance which must be achieved between the reduction in ink dot size and the stability of the ink jetting operation when small ink droplets are discharged, there is a certain limit upon the reduction in ink dot size as well. This places a limit upon improvements in image quality. Further, this technique is such that the higher the density, the greater the number of ink drops jetted for one pixel, as a result of which there is a decline in recording speed. This means that improvement in image quality and recording speed are mutually contradictory.
A known method of improving image quality without raising the integration density of the discharge ports is continuous tone recording using toned inks of the same color but of different ink concentrations. According to continuous tone recording, highlight portions are recorded using diluted ink, namely ink of low density, to make the graininess of the ink dots appear less conspicuous, and high-density portions are recorded using concentrated ink. As a result, high-density portions can be produced without doing so by increasing the number of ink drops discharged as in the multidrop method. This makes it possible to suppress a decline in recording speed. Further, with the continuous tone recording method, inks for expressing output image density signal level corresponding to input image density signal level are decided using a tone allocation table of the kind exemplified in FIG. 16.
FIG. 16 shows an example of a tone allocation table when inks of four colors (K, C, M, Y) are used and each ink is of three types (concentrated ink, medium ink and diluted ink).
As shown in FIG. 16, the tone allocation table indicates types of ink for expressing an output image density signal level that corresponds to an input image density signal level. When a recording operation is performed, the tone allocation table is used to develop an input image in conformity with ink for expressing an output image density signal level that corresponds to an input image density signal level. It should be noted that the tone allocation table is designed in dependence upon the percentage of pigment density in such a manner that the value of an input image density signal and the value of reflected density after recording will exhibit a proportional relationship.
When the type of ink which expresses the output image density signal level has been decided from the tone allocation table, the output image density signal level is subjected to binarization processing by a binarizing circuit to produce image signals Kconc, Kmed, Kdil, Cconc, Cmed, Cdil, Mcon, Mmed, Mdil, Yconc, Ymed, Ydill transferred to 12 multiheads.
In an image recorded by the arrangement described above, low-density areas such as image highlight portions are recorded using diluted ink so that the ink dots will not be too conspicuous, and high-density portions are recorded using medium ink and concentrated ink. As a result, image quality can be improved over that obtained with the multidrop method.
The dither method, error diffusion method and average density preservation method are known as known as methods of pseudo-halftone processing based upon binarization mentioned above.
The dither method binarizes the data of each individual pixel by a threshold value of each pixel decided by a dither matrix.
The error diffusion method, as described in "An Adaptive Algorithm for Spatial Gray Scale", by R. Floyd and L. Steinberg, SID 75 Digest, pp. 36.about.37, binarizes multilevel image data of a pixel of interest (i.e., converts the data to a maximum level or minimum level), calculates the error between a binarized level and the immediately preceding binarized level, diffuses the error to other pixels and adds the error.
The average density preservation method, as described in the specification of Japanese Patent Application Laid-Open No. 2-210962, obtains a threshold value based upon bilevel data obtained by binarization already performed in the vicinity of a pixel of interest, or data which includes the results of binarizing a pixel of interest to black or white, and binarizes the image data of the pixel of interest based upon this threshold value.
However, these conventional methods involve a number of difficulties. For example, in a situation where the output image is a transparency image such as X-ray film for medical purposes, visual resolution with respect to density is high. As a result, even if use is made of concentrated and diluted inks, a difference in density from one pixel to the next can be recognized and the impression that is given is one of a coarse image. In other words, it is necessary to increase further the number of tones of each pixel. In order to achieve this, however, it is necessary to increase the number of types of concentrated and diluted inks and to provide a correspondingly large number of recording heads conforming to the number of inks. The result is a considerable increase in cost.